Orbitally-driven Peierls state in spinels

We consider the superstructures, which can be formed in spinels containing on B-sites the transition-metal ions with partially filled t2g levels. We show that, when such systems are close to itinerant state (e.g. have an insulator-metal transition), there may appear in them an orbitally-driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that similar phenomenon should be observed in NaTiO2 and possibly in some other systems.Comment: 4 pages, 3 figure

Effective Hamiltonian of Three-orbital Hubbard Model on Pyrochlore Lattice: Application to LiV_2O_4

We investigate heavy fermion behaviors in the vanadium spinel LiV_2O_4. We start from a three-orbital Hubbard model on the pyrochlore lattice and derive its low-energy effective Hamiltonian by an approach of real-space renormalization group type. One important tetrahedron configuration in the rochlore lattice has a three-fold orbital degeneracy and spin S=1, and correspondingly, the effective Hamiltonian has spin and orbital exchange interactions of Kugel-Khomskii type as well as correlated electron hoppings. Analyzing the effective Hamiltonian, we find that ferromagnetic double exchange processes compete with antiferromagnetic superexchange processes and various spin and orbital exchange processes are competing to each other. These results suggest the absence of phase transition in spin and orbital spaces down to very low temperatures and their large fluctuations in the low-energy sector, which are key issues for understanding the heavy fermion behavior in LiV_2O_4.Comment: 26 pages, 26 figure

Exact spin-orbital separation in a solvable model in one dimension

A one-dimensional model of coupled spin-1/2 spins and pseudospin-1/2 orbitals with nearest-neighbor interaction is rigorously shown to exhibit spin-orbital separation by means of a non-local unitary transformation. On an open chain, this transformation completely decouples the spins from the orbitals in such a way that the spins become paramagnetic while the orbitals form the soluble XXZ Heisenberg model. The nature of various correlations is discussed. The more general cases, which allow spin-orbital separation by the same method, are pointed out. A generalization for the orbital pseudospin greater than 1/2 is also discussed. Some qualitative connections are drawn with the recently observed spin-orbital separation in Sr2CuO3.Comment: 5 page

Quantum Versus Jahn-Teller Orbital Physics in YVO3_3 and LaVO3_3

We argue that the large Jahn-Teller (JT) distortions in YVO$_3$ and LaVO$_3$ should suppress the quantum orbital fluctuation. The unusual magnetic properties can be well explained based on LDA+$U$ calculations using experimental structures, in terms of the JT orbital. The observed splitting of the spin-wave dispersions for YVO$_3$ in C-type antiferromagnetic state is attributed to the inequivalent VO$_2$ layers in the crystal structure, instead of the ``orbital Peierls state''. Alternative stacking of $ab$-plane exchange couplings produces the c-axis spin-wave splitting, thus the spin system is highly three dimensional rather than quasi-one-dimensional. Similar splitting is also predicted for LaVO$_3$, although it is weak.Comment: 4 pages, 2 tables, 2 figures, (accepted by PRL

The spin-wave spectrum of the Jahn-Teller system LaTiO3

We present an analytical calculation of the spin-wave spectrum of the Jahn-Teller system LaTiO3. The calculation includes all superexchange couplings between nearest-neighbor Ti ions allowed by the space-group symmetries: The isotropic Heisenberg couplings and the antisymmetric (Dzyaloshinskii-Moriya) and symmetric anisotropies. The calculated spin-wave dispersion has four branches, two nearly degenerate branches with small zone-center gaps and two practically indistinguishable high-energy branches having large zone-center gaps. The two lower-energy modes are found to be in satisfying agreement with neutron-scattering experiments. In particular, the experimentally detected approximate isotropy in the Brillouin zone and the small zone-center gap are well reproduced by the calculations. The higher-energy branches have not been detected yet by neutron scattering but their zone-center gaps are in satisfying agreement with recent Raman data.Comment: 13 pages, 5 figure

Variational Monte Carlo study of ferromagnetism in the two-orbital Hubbard model on a square lattice

To understand effects of orbital degeneracy on magnetism, in particular effects of Hund's rule coupling, we study the two-orbital Hubbard model on a square lattice by a variational Monte Carlo method. As a variational wave function, we consider a Gutzwiller projected wave function for a staggered spin and/or orbital ordered state. We find a ferromagnetic phase with staggered orbital order around quarter-filling, i.e., electron number n=1 per site, and an antiferromagnetic phase without orbital order around half-filling n=2. In addition, we find that another ferromagnetic phase without orbital order realizes in a wide filling region for large Hund's rule coupling. These two ferromagnetic states are metallic except for quarter filling. We show that orbital degeneracy and strong correlation effects stabilize the ferromagnetic states.Comment: 4 pages, 2 figure

Oxygen-stripes in La0.5Ca0.5MnO3 from ab initio calculations

We investigate the electronic, magnetic and orbital properties of La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure calculation within the Hartree-Fock approximation. Using the experimental crystal structure reported by Radaelli et al. [Phys. Rev B 55, 3015 (1997)], we find a charge-ordering stripe-like ground state. The periodicity of the stripes, and the insulating CE-type magnetic structure are in agreement with neutron x-ray and electron diffraction experiments. However, the detailed structure is more complex than that envisaged by simple models of charge and orbital order on Mn d-levels alone, and is better described as a charge-density wave of oxygen holes, coupled to the Mn spin/orbital order.Comment: 4 pages, 3 figures. Version accepted for publication in PR

Dimensional tuning of electronic states under strong and frustrated interactions

We study a model of strongly interacting spinless fermions on an anisotropic triangular lattice. At half-filling and the limit of strong repulsive nearest-neighbor interactions, the fermions align in stripes and form an insulating state. When a particle is doped, it either follows a one-dimensional free motion along the stripes or fractionalizes perpendicular to the stripes. The two propagations yield a dimensional tuning of the electronic state. We study the stability of this phase and derive an effective model to describe the low-energy excitations. Spectral functions are presented which can be used to experimentally detect signatures of the charge excitations.Comment: 4pages 4figures included. to appear in Phys. Rev. Lett. vol. 10

Mott insulating state in a quarter-filled two-orbital Hubbard chain with different bandwidths

We investigate the ground-state properties of the one-dimensional two-band Hubbard model with different bandwidths. The density-matrix renormalization group method is applied to calculate the averaged electron occupancies $n$ as a function of the chemical potential $\mu$. Both at quarter and half fillings, "charge plateaux" appear in the $n$-$\mu$ plot, where $d\mu/dn$ diverges and the Mott insulating states are realized. To see how the orbital polarization in the one-quarter charge plateau develops, we apply the second-order perturbation theory from the strong-coupling limit at quarter filling. The resultant Kugel-Khomskii spin-orbital model includes a $magnetic$ field coupled to orbital pseudo-spins. This field originates from the discrepancy between the two bandwidths and leads to a finite orbital pseudo-spin magnetization.Comment: 4 pages, 2 figures, Proceedings of LT2

Anharmonic effect on lattice distortion, orbital ordering and magnetic properties in Cs2AgF4

We develop the cluster self-consistent field method incorporating both electronic and lattice degrees of freedom to study the origin of ferromagnetism in Cs$_{2}$AgF$_{4}$. After self-consistently determining the harmonic and anharmonic Jahn-Teller distortions, we show that the anharmonic distortion stabilizes the staggered x$^{2}$-z$^{2}$/y$^{2}$-z$^{2}$ orbital and ferromagnetic ground state, rather than the antiferromagnetic one. The amplitudes of lattice distortions, Q$_{2}$ and Q$_{3}$, the magnetic coupling strengthes, J$_{x,y}$, and the magnetic moment, are in good agreement with the experimental observation.Comment: 13 pages, 5 figure